A large number of methods have conventionally been proposed for reducing amounts of information in recording image data obtained with image pickup devices such as TV cameras into storage devices such as memories.
Image compressing techniques which are typically represented by JPEG and MPEG are currently known as techniques for reducing amounts of image data. Images can be compressed on the order of 1/30 without remarkably degrading image qualities by using, for example, the MPEG technique.
Further, a data reducing technique adopted for an image recorder disclosed by Japanese Patent Application Laid-Open No. 63-217786 can be mentioned as another technique for reducing amounts of image data. This image recorder is configured, as shown in FIG. 10, to lead a video signal 1 picked up by a monitor camera or similar means into a comparison input of a comparator circuit 7, and into a subrecording block 5 and a main recording block 12 through a switch 4 and another switch 6. Furthermore, the video signal is added from the sub-recording block 5 to a standard input of the comparator circuit 7 and an output from the comparator circuit 7 is input into a control block 2. The control block 2 is caused to control the switches 4 and 6 as well as readout from the sub-recording 5 by inputting a synchronous signal which is extracted by a synchronous extraction circuit 13, into the control block 2. An image signal picked up by the sub-recording block 5 is recorded temporarily as a previous value, the recorded image signal is compared with an image signal which is subsequently picked up and an image signal differentiated according to a comparison result is recorded in the main recording block 12. A recording capacity of the main recording block is reduced as described above.
A data reducing technique used in a theft-preventive monitor disclosed by Japanese Patent Application Laid-Open No. 7-231442 can be mentioned as still another technique for reducing amounts of image data. In this theft-preventive monitor, as shown in FIG. 11, input image data from video camera 1 of a theft-preventive unit, which is installed at a location under the monitor, is input into a motion judging block 2 and an image encoding block 3. The motion judging block 2 detects motion data and judges whether or not an object under the monitor moves. Only when the motion judging block 2 judges that the object moves, the image encoding block 3 outputs image data reproduced by a local image decoding block 4 to a data recording block 5 for recording the image data. Only when the object under monitor moves, the image encoding block 3 transmits encoded data to the theft-preventive unit disposed at the location under monitor. An abnormal event judging block 6 receives this data and generates an alarm signal when it judges the movement as an abnormal event. Simultaneously, an image decoding block 8 decodes the encoded data, and a recording block 7 on a monitor side records an image and displays the image on a monitor block 9. The theft-preventive monitor reduces amounts of image data to be recorded as described above.
However, the known image detectors have defects which are described below.
First, the method which reduces amounts of image data using the compressing technique represented by MPEG must process data in an amount of 4 Mbps even in a compressed condition on the order of 1/30 since an original amount of image data is remarkably large and requires, for example, a memory of approximately 43 G bytes for recording the data continuously for a period of 24 hours. Such a large memory requirement hardly allows an image detector to be manufactured at a low cost using a random access memory under the present conditions.
Further, continuity in time is lost in data recorded by a method that records only differentiated images or moving images like those adopted for the image recorder as shown in FIG. 10 and disclosed by Japanese Patent Application Laid-Open No. 63-217786, or the theft-preventive monitor as shown in FIG. 11 and disclosed by Japanese Patent Application Laid-Open No. 7-231442. Although the method effectively reduces amounts of image data. The importance of images on a divalent basis, i.e., whether or not the images are different from each other or whether or not an image moves, presents a problem in that it may allow important data to be effaced when an attempt is made to record new data while automatically effacing data in the event where a capacity of the recording medium is exceeded.